It is often desired to reclaim or recycle vulcanized rubber. The vulcanized rubber is generally in the form of a manufactured article such as a pneumatic tire, industrial conveyor or power transmissions belt, hose and the like. Scrap pneumatic tires are especially large source of such vulcanized rubber.
The vulcanized rubber is conventionally broken down and reclaimed or recycled by various processes, or combination of processes, which may include physical breakdown, grinding, chemical breakdown, devulcanization and/or cryogenic grinding. If the vulcanized rubber contains wire or textile fiber reinforcement, then it is generally removed by various processes which might include a magnetic separation, air aspiration and/or air floatation step.
In this description, the terms "recycle" and "recycled rubber" are used somewhat interchangeably and relate to both vulcanized and devulcanized rubber which is more completely hereinafter described. It is important to appreciate that devulcanized recycle or recycled rubber (sometimes referred to as reclaim rubber) relates to rubber which had been vulcanized followed by being substantially or partially devulcanized.
The resultant recycle rubber that had been devulcanized is a polymeric material which has somewhat the appearance of unvulcanized rubber but has important differences and properties therefrom. First, it is a rubber which is, in essence, a partially vulcanized rubber composed of a mixture of polymer units of various and numerous constructions different from either unvulcanized or vulcanized rubber.
Secondly, the recycled rubber, unlike conventional unvulcanized rubber, is also a complex mixture of largely unknown polymer(s), of compounding ingredients, possibly bits of textile fiber, and the like.
It has been observed that, after adding sulfur and accelerator to recycle rubber, followed by its revulcanization, the resulting physical properties, such as tensile and elongation, are usually lower than the corresponding properties of the original vulcanized rubber from which it was derived. It has also sometimes been observed that exposed edges of bales or slabs of recycle rubber have tended to curl up, apparently a result of oxidation degradation which was probably due to a deficiency of antidegradants which would normally have been adequately present in unvulcanized, compounded rubber.